GERM LAYERS. 67 



homologous with the . simpler protentoderm formation from the crescentic 

 groove (blastopore) in Reptiles. (Compare Fig. 51 with Fig. 42.) As the 

 protentoderm grows forward in the medial line it apparently replaces the yolk 

 entoderm, so that the roof of the new cavity the archenteron is formed of 

 protentoderm. The area where the protentoderm fuses with the yolk entoder 

 is, as in Reptiles, the "completion plate." 



The only real difference between gas'trulation in Reptiles and in Birds i 

 that in Birds the crescentic groove (original blastopore) becomes transforme 

 into the primitive groove which remains open only at its anterior end (Hensen 

 node). In Reptiles the blastopore may be of any form, crescentic, round, ova! 

 etc., but does not become transformed into a longitudinal linear structure; so [:, 

 this case the primitive intestinal invagination (the head process, "primitive 



.^.^^^^ -Area opaca 

 Area pellucida 



^^HM9i^^^' <H& 



"Completion plate' 

 Head process 

 -Medullary folds 

 Hensen's node- 



Primitive streak 



. FIG. 50. Surface view of chick blastoderm. Bonnet. 



intestinal cord") grows forward from the original point of invagination at the 

 posterior margin of the disk. 



Gastrulation in Mammals. 



Reference to the description of segmentation in the mammalian ovum and 

 its peculiarities (p. 51) makes it evident that these peculiarities must deter- 

 mine further modifications in the development of the germ disk as compared 

 with lower forms. It will be remembered that segmentation in the mamma- 

 lian ovum had been carried to the differentiation of two kinds of cells (p. 52), 

 an outer cell layer (trophoderm) and an inner cell mass (Fig. 33). In lower 

 forms the first cell differentiation came with the formation of the two primary 

 germ layers, the ectoderm and the entoderm, and these with the enclosed cavity 

 constituted the gastrula. The first cell differentiation in Mammals has, how- 

 ever, an entirely different significance, the trophoderm having nothing to 

 do with the formation of the embryo but being destined to give rise to extra- 

 embryonic structures. It is the cells of the inner cell mass or embryonal bud 



